1. Field of the Invention
The present invention relates to the field of electroplating.
2. Prior Art
Electroplating methods and apparatus are very well known in the prior art, and are used on an everyday basis for such purposes as applying protective and decorative metal platings or coatings to a wide variety of metal products, including stampings, castings, extrusions and the like. Various methods are also known for electroplating nonconductive materials, such as by way of example, plastics, by first applying a thin conductive layer so that the electroplating process may be utilized. Such initial conductive layers may be applied by such means as electroless plating techniques, by coating with a conductive paint or similar material, or even by the use of a thin metal plating deposited by vapor deposition techniques.
Prior art electroplating methods and apparatus utilize a bath of an electrolyte which is rich in the ions of the metal to be plated. The parts to be plated are immersed in this electrolyte bath and are electrically coupled to a cathode or negative terminal of an appropriate electrical power supply. Typically also immersed in the electrolyte are one or more bars of the metal to be plated which in turn are electrically coupled to the positive terminal of the power supply. Basically some of the metal ions in the electrolyte adjacent the parts to be plated deposit onto the part and are electrically neutralized, with the ions being replaced at the anode by the gradual ionization and passage into solution of the anode metal. Thus a prior art electroplating system has a closed electrical circuit which includes not only the part being plated and the anodes of the plating metal but also the path of electrolyte therebetween, with the flow of the electrical current in the electrolyte being a result of the flow of metal ions from the anode to the part being plated. Thus the rate of plating in prior art systems is limited by the rate at which the metal ions may reasonably be caused to flow through the electrolyte and provide a good quality plated surface, a factor which is particularly limited on articles having sharp edges or projections, as points, edges, and the like tend to concentrate the electric field at that region to cause local burning or a low quality plating on the article being plated. Accordingly plating rates in prior art systems are limited, and are particularly limited in plating rates for articles having protruding edges, corners and the like.
The plating of articles having holes, depressions and the like is also highly limited in prior art plating systems. In an unagitated electrolyte, the metal ions proceed through the bath in accordance with the electric field therein. Since the electric field surrounding a hole is highest in the region of the mouth of the hole, and grossly diminishes along the length of the hole, the electrolyte within the hole becomes starved for the metal ions, with the result that the plating rate therein is very low compared to the plating rate at the mouth of the hole and at other regions of the part being plated.
One method used in prior art electroplating equipment for increasing the plating speed and obtaining better plating of holes and other shaded areas on the article being plated is to agitate the electrolyte by such means as pumps, mixers and the like, or the passage of air therethrough. This agitation or circulation mechanically aids in the transport of the ions from the anode to the part being plated, and also aids in the penetration of the ion rich electrolyte into holes and depressions in the article being plated. However, characteristically high agitation of the bath is required to cover all areas of the items being plated, with the net result that plating rates in holes and depressions are still substantially lower than on direct flat surfaces. By way of example, plating rates in the through holes in printed circuit boards are on the order of fifty percent or less of the plating rate achieved on the board face. Also the use of air as opposed to other means for circulating the electrolyte tends to decrease the plating rate because of the fact that the air displaces the electrolyte and diminishes the transport of the metal ions by the electric field.
Typical plating solutions often contain at least small amounts of organic compositions and/or other non-conductive constituents which, if deposited or allowed to accumulate on the surface of the article being plated, will interupt the plating at that local region. While agitation of the electrolyte tends to diminish this effect it does not always eliminate such accumulations.
Agitating methods and apparatus for agitating the electrolyte in electroplating tanks is well known, as exemplified in U.S. Pat. Nos. 593,837; 1,431,022; 3,503,856 and 3,963,588. All of these methods envision forms of spray apparatus for positioning below the electrolyte level, with the spent spray passing into the bulk of the tank, though the last of these patents contains the solitary statement that such immersion is not a requisite to effective electroplating, and that the electrolyte issuing from the housing may simply be conducted to a reservoir tank and recycled, as necessary, back to the inlet.
Other systems for delivering an electrolyte to the surface to be plated through the use of some form of pump arrangement are also known. By way of example, in the system disclosed in the British Pat. No. 986, a pump is used to deliver an electrolyte through a tube C directly onto the face of the article to be plated. It is apparent from the disclosure in that patent, however, that the entire article is immersed in the electrolyte, as the return line is placed above both anode and cathode. In French Pat. No. 763,863 a system for spot plating is disclosed wherein an electrolyte is allowed to flow downward onto the local region to be plated, with the electrolyte then running off to the collection region therebelow. Also, the use of spraying apparatus in various forms is known with respect to non-electrical processes, such as in etching (U.S. Pat. No. 2,895,814) and in treatment liquids of various kinds (U.S. Pat. No. 3,824,137). Finally, anode containers of various kinds are also known such as those disclosed in U.S. Pat. No. 3,300,396.